Inflammatory Bowel Disease (IBD) affects more than one million Americans and is characterized by visceral hypersensitivity (VH), the mechanisms of which are not fully understood. Microglia have been implicated in neuropathic pain and VH, with increased degrees of activation correlating with increased hypersensitivity;however, their role in visceral pain has been less studied. This project will be the first in which the functional properties of microglia will be investigated in sensitized tissues of the dorsal horn in the context of VH. The long-term objective is to define these functional properties so that they may be targeted therapeutically in IBD. The hypothesis of this study is that colon inflammation-induced microglial activation contributes to VH, which is diminished by reversion of microglia to a less reactive state. This hypothesis will be tested in two specific aims: 1) To demonstrate a shift in microglial morphology from a surveillance state to a reactive state characterized by increased expression of CDIIb and Ibal proteins and membrane purinergic receptors (P2X4 and P2Y12) and 2) To show that colon-inflammation induced visceral hypersensitivity is reversed by inhibiting microglia with minocycline, causing them to revert back to a more surveillance-like state. Colon inflammation will be induced in adult rats using intracolonic administration of 2,4,6- trinitrobenzenesulfonic acid (an established model in the lab). VH will be evaluated in rats with colon inflammation and naive rats using electromyography. Cellular markers and protein expression will be evaluated in both groups using immunohistochemistry, Western blotting, and real-time PCR. Since neuronal hyperexcitability is associated with VH, extracellular action potentials from spinal cord neurons will be recorded to determine whether there is a correlation between neuronal hyperexcitability and microglial activation. PUBLIC HEALTH RELEVANCE: Morbidity associated with IBD is substantial, with symptoms such as pain, diarrhea, bloody stools, infection caused by colon ulcerations, and malnutrition, and often results in colon removal. In the United States, it is estimated that IBD costs over two billion dollars annually, including medical care and lost productivity. This research will potentially broaden treatment options available for IBD, which may also extend to treatment of functional gastrointestinal disorders like irritable bowel syndrome.